When people communicate with each other over a distance, there is often uncertainty as to whether the participants hear each other. The uncertainty can be attributed to noise or other forms of impairment that are present in the transmission path between the participants. The uncertainty in achieving successful communication is not specific to any one form of communication. This is evident, for example, in two-way radio communication for military, aerospace, or public service purposes, in which the participants are often forced to repeat what was heard. Additionally, in order to acknowledge and confirm the accuracy of the communication explicitly, the participants often have to use words such as “roger” to indicate that information has been received or “copy” to indicate that what was just said is understood. Even in cellular telecommunications, which comprises an adapted form of two-way radio communication, commercials and advertisements by the cellular service providers often feature themes of anxiety caused by a cell phone user not hearing someone else or of a service rep continually asking on his cell phone if someone else is able to hear him.
As alluded to above, radio communication is particularly vulnerable to impairments, which can come in the form of co-channel interference, Rayleigh fading, thermal noise, and so forth. Often, there is only one person who is using a wireless phone on a call, the other person being on a landline phone such as an office deskset. Increasingly, however, a call that involves a first wireless phone user will also involve a second wireless phone user. The scenario of two wireless phones can involve a cell phone and WiFi device, two cell phones, two WiFi devices, and so forth.
A wireless user often has some kind of signal strength indication that is typically based on measurements of a received signal's strength, which indication is then displayed at the phone. Sometimes, however, the indication suggests that a service provider's coverage area is better than it actually is. This can occur, for example, when a service provider boosts the power that is transmitted by the serving base station over the downlink to a wireless phone, leading a wireless user to think that the call quality is good, even though the carrier-to-impairment ratio of the downlink might still be unsatisfactory. In any event, the corresponding uplink might turn out to be far weaker or noisier, and so the wireless user does not know with any reasonable certainty whether the far-end party is hearing what is being said. Furthermore, if there are two wireless terminals involved in the call (i.e., one for each party), the first party really has only a rough idea, and probably an unreliable one, of what the far-end party is receiving, based on the displayed signal-strength indication.
Not surprisingly, a lot of effort has been spent on improving the robustness of communications and ensuring each participant in a conversation that the other person is, in fact, hearing him. In a Voice over Internet Protocol context, for example, protocols such as Real-time Transport Control Protocol (RTCP) have been developed to deal with improving the robustness of communication, among other things. RTCP in particular provides out-of-band control information for a Real-time Transport Protocol (RTP) packet stream that conveys voice or other media information between terminals. The receiving terminal of a packet stream can transmit RTCP packets back to the transmitting terminal, in order to report the quality of service being experienced by the receiving terminal. However, this is disadvantageous because it requires either additional standardization across terminals or additional transmission infrastructure, or both.